Permanent magnet alloy



tamination. so that 0.05%

PERMANENT MAGNET ALLOY Dennis Hadfield, Sheffield, England, assignor to William Jessop & Sons Limited, Sheffield, England No Drawing. Application July 10, 1951, I

Serial N0. 236,053

5 Claims. (Cl. 317---202) This invention relates to permanent-magnet alloys of the Alnico type (Fe-Ni-Al-Co-Cu) to which anisotropic characteristics have been imparted, and is a continua- "ice have been subjected to similar thermo-magnetic, thermal,

and magnetic treatments required to achieve anisotropic characteristics.

According to the present invention, a magnetically I anisotropic permanent magnet consists of an alloy of the Fe-Ni-Al-Co-Cu type containing about 12.5% to 14.5% nickel, 7.5% to 8.5% aluminium, 23.5% to 25.5% cobalt, 2.9% to 3.3% copper, less than 0.05% carbon, about 1 /2% columbium with about 1 /2% tantalum (all within the usual manufacturing tolerances), with the balance mainly iron, said magnet having in a preferred tion-in-part of U. S. patent application Serial No. 591,183,

filed November 9, 1948, now abandoned.

The method of producing anisotropic permanent magnets as now largely practised has its basis in work by D. A. Oliver and I. W. Shedden, who, as they set out in a letter published in the issue of Nature dated July 30, 1938 (p. 209), subjected a permanent magnet alloy of the Alnico type to the action of a magnetic field whilst cooling from a high temperature such as, for example. l700 C. Since that date, such treatment has been applied on a large scale to magnet alloys capable of attaining anisotropic characteristics, and, although various additionsnotably of titaniumto the basic Fe-Ni-Al-Co-Cu alloy have been proposed in endeavours to achieve still better magnetic properties, practice for some years past has been lar ely confined to this basic alloy, since it appeared that that alloy yielded the best results in the present state of knowledge.

Even prior to the publication of the work by Oliver and Shedden, numerous additions had been proposed to Alnico alloys to improve their properties (then, of course, isotropic) and/or to facilitate their manufacture: and, since the present invention is based on the discovery that very advantageous results may be achieved with anisotropic alloys of the type in question by the addition of columbium in coniunction with tantalum, it is relevant to mention that British patent specification No. 496,774 (dated November 5, 1937, and accepted December 6. 1938) described the addition of columbium to an isotropic Alnico permanent magnet alloy for the purpose of reducing the harmful effects of contamination by carbon to the ma netic properties of the alloy.

Since 1937, the general adoption of induction furnaces for melting magnet allovs has much reduced carbon concarbon would rank as a high amount of this impurity; 0.02% to 0.03% is the maximum now found in the best commercial alloys. It is. therefore, very doubtful if this smaller amount of carbon has any noticeable depressing effect on the magnetic properties, but British specification No. 496,774 shows that as little as 0.2% to 0.3% columbium would be ample to neutralise it.

As indicated above, the present invention makes use of columbium in coniunction with tantalum in anisotropic permanent magnet alloys of the Fe-Ni-Al-Co-Cu type, and the obiect of the invention is to obtain permanent ma nets of this type having a hi h coercive force in the preferred direction. and particularlv havin a combination of high coercive force and high (BHlmax. better than has hitherto been obtainable in magnets of such high coercivity, except by the use of columbium itself-as is described and claimed in co-pending U. S. patent application Serial No. 236,052 of even date herewith, itself a continuation-in-part of U. S. patent application Serial No. 59,183.

When a permanent magnet alloy is mentioned in the present specification, it is to be understood that this term applies alike to a casting produced by melting the constituent elements together and to a sintered mass produced by subjecting a powdered mixture of those elements to pressure in a mould and heating the compact, it having been found that such a sintered mass behaves very similarly to a casting of like composition when both direction a coercivity of at least 800 oersted and a (BH)max. of at least 3.8)(10 gauss-oersted, the product of coercivity and (BH)max. being at least 3.0X10

For practical purposes, the four first constituents may -approximate to 13.5% nickel, 8% aluminium, 24.5%

cobalt, and 3% copper.

In an example according to the invention an alloy proved to have the following check analysis:

Patented Nov. 9, 1954 Ni Al 00 Cu Cb Ta Balance 13. 4% 7.75% 25.0% 3.0% 1.53% 1.99% (0 less Fe etc.

than0.05%)

was cast to form a test piece, 40 35 10 mm., which, after being ground on all faces to obtain an accurate determination of magnetic properties, was cooled at 1.4 C./sec. from a solution temperature of 1250 C. to below the Curie point, aged at 590 C. for 48 hours followed by 560 C. for 48 hours, magnetised, and tested. The magnetic properties were:

ram. H0 )mlxgauss oersted gauss-oersted The cooling rate is not critical. Thus, instead of the approximate 1.5 C./sec. of the above example, l.0

C./sec. and 0.5 C./sec. may be used, or, in general, a cooling rate within the range 0.2 C./sec. to 5 C./sec.

Generally, the slower the cooling rate the higher the coercivity but the lower the remanence and (BH)max. whilst the faster the cooling the higher the remanence and (BH)max. but the lower the value of the coercivity. Consequently it is desirable to use the slower cooling rates for very short magnets and to increase the rate for longer magnets.

Further, it is generally advisable to reduce the cooling rate when the nickel content is reduced and to increase it when the aluminium content is reduced and vice versa.

For the purposes of the present invention, carbon, chromium, manganese, molybdenum, phosphorus, and silicon, silver, sulphur, titanium, tungsten, vanadium, or zirconium, are regarded as impurities (included, if present, with the balance of Fe etc.) and should not be present in amounts exceeding a total of 1%. Carbon should be (and is, if induction heating is used) kept lowbelow 0.05

What I claim is:

1. A magnetically anisotropic permanent magnet, consisting of an alloy of the Fe-Ni-Al-Co-Cu type containing about 12.5% to 14.5% nickel, 7.5% to 8.5% aluminium, 23.5% to 25.5% cobalt, 2.9% to 3.3% copper, less than 0.05% carbon, 1V2% columbium, and 116% tantalum (all within the usual manufacturing limits), with sisting of an alloy of the Fe-Ni-Al-Co-Cu type'containingi about 13 .5% nickel, 8% aluminium, 2.45% cobalt, 3% copper, less than 0.65% carbon, l /2%- columbium; and. l/2% tantalum. (all Within the usual manufacturing, limits), with the balance mainly iron; said* magnet hav ing in a preferred: direction a coercivity of at least 800' oersted, and a (BH)max. of at least 3.8x 1-0 gaussoersted;. the product of the coercivity and: the (BH)'-mx. of the magnet being at least 3.0 X10 3. A magnetically anisotropic permanent magnet, con sisting of an alloy of the Fe-Ni-Al-o-Cu type containing about 13.4% nickel, 7.75% aluminium, 25.0% cobalt, 3.0% copper, less than0.05% carbon, 1.531% columbium, and 1.99% tantalum (all within the usual manui'actur ing tolerances), with the balance mainly iron; said mag-- a l net having in a preferred direction a coercivity of about 855 oersted, anda (B-H)max. of about 4.75X 1-9 gaussoersted.

4. A magnetically anisotropic permanent magnet consisting of an alloy of the Fe-Ni-Al-Co-Cu type containing about 13.5% nickel, 8% aluminium, 24.5% cobalt, 3%

copper, less than 0.5% carbon, 1'V2% columbium, and

1V2% tantalum, with the balance substantially all iron.

5. A magnetically anisotropic magnet consisting of an alloy of the Fe-Ni-Al-Co-Cu typev containing about 13.5% nickel, 8% a1uminum,.24.5% cobalt, 3% copper, 1V2% colurnbiurn and l /z% tantalum, with the balance substantially all iron.

References Cited in the file of this patent FOREIGN PATENTS Great Britain June 26', I940, 

5. A MAGNETICALLY ANISOTROPIC MAGNET CONSISTING OF AN ALLOY OF THE FE-NI-AL-CO-CU TYPE CONTAINING ABOUT 13.5% NICKEL, 8% ALUMINUM, 24.5% COBALT, 3% COPPER, 11/2% COLUMBIUM AND 11/2% TANTALUM, WITH THE BALANCE SUBSTANTIALLY ALL IRON. 